Signals carried over communication channels can undergo considerable transformations, such as digitisation, encryption and modulation. They can also be distorted due to the effects of lossy compression and transmission errors.
Objective processes for the purpose of measuring the quality of a signal are currently under development and are of application in equipment development, equipment testing, and evaluation of system performance.
A number of patents and applications relate to this field, most particularly European Patent 0647375, granted on 14 Oct. 1998. In this invention two initially identical copies of a test signal are used. The first copy is transmitted over the communications system under test. The resulting signal, which may have been degraded, is compared with the reference copy to identify audible errors in the degraded signal. These audible errors are assessed to determine their perceptual significance—that is, errors that are considered significant by human listeners are given greater weight than those that are not considered so significant. In particular inaudible errors are perceptually irrelevant and need not be assessed.
The automated system provides an output comparable to subjective quality measures originally devised for use by human subjects. More specifically, it generates two values, YLE and YLQ′, equivalent to the “Mean Opinion Scores” (MOS) for “listening effort” and “listening quality”, which would be given by a panel of human listeners when listening to the same signal. The use of an automated system allows for more consistent assessment than human assessors could achieve, and also allows the use of compressed and simplified test sequences, which give spurious results when used with human assessors because such sequences do not convey intelligible content.
In the patent specification referred to above, an auditory transform of each signal is taken, to emulate the response of the human auditory system (ear and brain) to sound. The degraded signal is then compared with the reference signal in the perceptual domain, in which the subjective quality that would be perceived by a listener using the network is determined from parameters extracted from the transforms.
Such automated systems require a known (reference) signal to be played through a distorting system (the communications network or other system under test) to derive a degraded signal, which is compared with an undistorted version of the reference signal.
Such systems are known as “intrusive” measurement systems, because whilst the test is carried out the system under test cannot, in general, carry live traffic.
Co pending European Patent Application EP 00304497.1 describes a method and apparatus for measuring the performance of a communications channel whilst in use by exploiting periods of spare capacity. The method requires changes to both the transmitter and receiver—the transmitter must be modified in order to identify periods of spare capacity and insert the test signal; the receiver must be modified to extract the received frames prior to a source decoder or prior to a channel decoder, depending upon whether raw error patterns or residual error patterns are required.